The present invention relates to an X-ray image diagnostic apparatus for obtaining an X-ray image of a diagnosed part of an object to be examined by detecting X-ray that has transmitted through the object in a flat panel detector. More particularly, it relates to an X-ray image diagnostic apparatus for obtaining an image that has no motion artifact by removing lags in the flat panel detector.
In an X-ray image diagnostic apparatus hitherto, an object to be examined is irradiated with X-ray and the X-ray that has transmitted through the diagnosed part of the object is received with an X-ray detector, such as a flat panel detector (refer to Paul R. Granfors; xe2x80x9cPerformance Characteristics of an Amorphous Silicon Flat Panel X-ray Imaging Detector.xe2x80x9d Proc. SPIE Medical Imaging, February 1999. pp. 480xcx9c488), and then the X-ray image outputted from said X-ray detector is displayed on a CRT (cathode-ray tube) monitor or the like.
A flat panel detector used in an X-ray image diagnostic apparatus is comprised of a scintillator to transform X-ray that has transmitted through the object into light, a photo diode (an amorphous silicon type, for example) to transform the light outputted from said scintillator into electrical charge. This flat panel detector obtains an X-ray image by reading out electrical charge of the photo diode through a switching element (TFT(Thin Film Transistor), for example).
In the flat panel detector mentioned above, it is known that even after reading out electrical charge from the photo diode, the charge that is not read out remains in the photo diode. FIG. 5 is an example showing how the electrical charge, or the lags, remaining in the photo diode decreases along with time. (For the detail, refer to P. L. Weisfield; xe2x80x9cHigh Performance Amorphous Silicon Image Sensor for X-ray Diagnostic Medical Imaging Applications.xe2x80x9d Proc. SPIE Medical Imaging, February 1999. pp. 307xcx9c317.) As is clear in FIG. 5, the amount of the electrical charge remaining in the photo diode hardly changes in a short time within a second for taking images of fluoroscopy and radiography. Then the remaining charge becomes lags that affect the next image and a motion artifact, which is an obstacle to diagnosis, appears on the image.
For removing the lags of the previous image, the electrical charge remaining in the photo diode of the flat panel detector is read out without memorizing it into an image memorizing means before memorizing the next image from the flat panel detector into the image memorizing means (hereafter, it is referred to as xe2x80x9creadout with no recording.xe2x80x9d)
However, in the conventional art mentioned above, for removing the lags of the previous image, readout with no recording must be performed to all photo diode of the flat panel detector, that is, to all pixels. Then said readout takes as much time as performing readout to one more image.
Also, when there exists a region with an extremely high signal level in an X-ray image, the lags are not sufficiently removed by performing readout with no recording only one time. Then, readout with no recording for several images has to be performed.
FIG. 6 is a figure for explaining an example of the above case and FIG. 6(a) is an example of a histogram of electrical charge stored in the photo diode of the flat panel detector that has received the X-ray image. In the FIG. 6(a), there exists a region with an extremely high signal level in the center of the X-ray image. FIG. 6(b) shows the histogram of electrical charge stored in the photo diode of the flat panel detector after performing readout with no recording one time to the X-ray image of FIG. 6(a). In said histogram, there still remains a region with a high signal level. Then, for removing the lags of said high signal level part, readout with no recording for several times has to be performed.
The present invention is made concerning said circumstances. The purpose of said invention is to shorten the time for the readout with no recording by performing the readout with no recording only to the pixels having high level lags of the previous image, as well as to provide an X-ray image diagnostic apparatus to obtain an image of high quality by removing motion artifact generated due to said lags.
To achieve said purposes, the X-ray image diagnostic apparatus of the present invention comprises:
an X-ray source for irradiating X-ray to the object to be examined;
a flat panel X-ray detector for detecting X-ray that has transmitted through said object and for generating X-ray image data, said flat panel X-ray detector being arranged opposite to said X-ray source;
image memorizing means for memorizing the X-ray image that is output above as image data;
display means for displaying the image data that is output above as an image;
operation means for outputting the start signal to read out the image; and
readout control means for controlling the readout of the X-ray image data from said flat panel X-ray detector when receiving the start signal to read out the image,
wherein said operation means comprises means for determining its readout range by the histogram of remaining data after reading out X-ray image data obtained at the previous time phase of the predetermined time phase from said flat panel X-ray detector to said image memorizing means. And said readout control means comprises means for controlling readout of the remaining data in said flat panel X-ray detector from the readout range that is determined above.
And in the X-ray image diagnostic apparatus comprising:
a console for outputting a start signal to read out the image;
a flat panel detector for receiving an image of X-ray that has transmitted through the object to be examined and outputting said X-ray image;
image memorizing means for memorizing an X-ray image outputted from said flat panel detector as image data;
display means for displaying the image data memorized in said image memorizing means as an X-ray image; and
readout control means for controlling readout of the X-ray image data from said flat panel detector when receiving the start signal to read out the image outputted from said console,
said apparatus further comprises:
previous image memorizing means for memorizing the image data of an X-ray image outputted from said flat panel detector, as well as for performing readout with no recording to the particular pixels of said flat panel detector before said flat panel detector outputs the next image data and overwriting said image data to which readout with no recording has been performed; and
readout address determining means for extracting the particular pixels from the image data memorized in said previous image memorizing means and for calculating the address corresponding to the position of said particular pixels and outputting it to said readout control means. Said particular pixels are extracted according to the signal level of the image data, and said readout control means performs control in order to perform readout with no recording only to the particular pixels of said flat panel detector in accordance with the address of the particular pixels inputted from said readout address determining means.
Compared with the conventional X-ray image diagnostic apparatus, the apparatus having said structure, to which previous image memorizing means and readout address determining means is added, can extract the particular pixels to which readout with no recording is performed according to the signal level of the image data, as well as overwrite of the image data that is read out with no recording from the particular pixels of previous image memorizing means. Then, the condition of remaining electrical charge at each pixel in the flat panel detector before and after readout with no recording can be grasped. Besides, as the address of the particular pixels to which readout with no recording is performed can be determined in readout address determining means, readout with no recording only to the particular pixels can be performed through readout control means. Consequently, as readout with no recording is performed only to the particular pixels, time for readout with no recording can be shortened.
Also in the X-ray image diagnostic apparatus of the present invention, the particular pixels of said previous image memorizing means is the pixels having a signal level equal to or above a particular signal level (hereafter referred to as a threshold.) In said structure, among pixels in the flat panel detector the particular pixels to which readout with no recording is performed are extracted by using a particular signal level as the threshold. Then the pixels having a high signal level are read out with no recording. With said readout with no recording, readout of electrical charge begins from pixels having much remaining charge in the flat panel detector. And so remaining charge is efficiently reduced.
Also in the X-ray image diagnostic apparatus of the present invention, extraction of the particular pixels of said previous image memorizing means and readout with no recording of the particular pixels of said flat panel detector are performed for a plural number of time. In said structure, extraction and readout with no recording of the particular pixels is performed in the flat panel detector is executed more than two times if electrical charge remaining in the flat panel detector is not removed sufficiently by executing readout with no recording only one time. Consequently, remaining charge in the flat panel detector is removed sufficiently and efficiently.
Also in the X-ray image diagnostic apparatus of the present invention, the threshold used as a standard for extracting the particular pixels to which readout with no recording after the second time is performed from the image data of said previous image memorizing means is determined from the previous threshold. In said structure, as the threshold for the standard of extracting the particular pixels to which readout with no recording is performed can be determined in relation to the previous threshold, the threshold can be determined easily by, for example, keeping the ratio between the two thresholds. Also, as the threshold is determined in relation to the signal level of the pixels of the flat panel detector, the condition of electrical charge remaining in the flat panel detector can be grasped easily and the remaining charge can be also reduced certainly.
Also in the X-ray image diagnostic apparatus of the present invention, the number of the particular pixels to which readout with no recording is performed is subtracted from the number of pixels for one frame, and the resultant value of said subtraction is used as the particular pixels to which readout with no recording after the second time is performed from the image data of said previous image memorizing means. In said structure, readout with no recording can be performed two times while reading out the image data for one frame. For example, readout with no recording can be performed efficiently by reading out pixels having a signal level equal to or above the threshold in the first readout with no recording and then reading out the image data in descending order of the signal level in the second readout with no recording.